An inflator for inflating an inflatable vehicle occupant protection device, such as an air bag, contains a body of ignitable gas generating material. The inflator further includes an igniter. The igniter is actuated so as to ignite the body of gas generating material when the vehicle experiences a collision for which inflation of the air bag is desired to protect the vehicle occupant. As the body of gas generating material burns, it generates a volume of inflation gas. The inflation gas is directed into the air bag to inflate the air bag. When the air bag is inflated, it expands into the vehicle occupant compartment and helps to protect the vehicle occupant.
Ammonium nitrate based gas generating compositions for generating gas to inflate an inflatable vehicle occupant protection device have the advantage that they produce non-toxic nitrogen gas during combustion. At the same time, ammonium nitrate based gas generating compositions have some limiting drawbacks. For example, ammonium nitrate has a very low crystal phase stability and passes through five distinct crystal phase changes in the temperature range of about -17.degree. C. to about 169.degree. C. The crystal phase changes cause changes in the density and the volume of the particles of ammonium nitrate. The changes in the density and the volume can result in destruction of the grain integrity of the gas generating composition.
The most disadvantageous crystal phase change of ammonium nitrate is the Phase IIIPhase IV change which occurs at ambient temperature, specifically about 32.3.degree. C. This Phase IIIPhase IV change is characterized by a significant irreversible change in crystal volume of the ammonium nitrate. The crystal phase change at ambient temperature or 32.3.degree. C. can be inhibited by stabilizing the crystal structure of ammonium nitrate with a phase stabilizer such as potassium nitrate.
Nevertheless, ammonium nitrate is hygroscopic and readily absorbs moisture. Moisture can cause hydrolysis of the phase stabilizer and loss of the phase stability. The presence of moisture within the ammonium nitrate also makes the ammonium nitrate difficult to ignite. Accordingly, it may be desirable to examine the ammonium nitrate to ensure that the moisture level within the ammonium nitrate is not above a predetermined level.